A sub-wavelength Si LED integrated in a CMOS platform

A nanoscale on-chip light source with high intensity is desired for various applications in integrated photonics systems. However, it is challenging to realize such an emitter using materials and fabrication processes compatible with the standard integrated circuit technology. In this letter, we report an electrically driven Si light-emitting diode with sub-wavelength emission area fabricated in an open-foundry microelectronics complementary metal-oxide-semiconductor platform. The light-emitting diode emission spectrum is centered around 1100 nm and the emission area is smaller than 0.14 μm2 (~\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varnothing 400$$\end{document}∅400 nm). This light-emitting diode has high spatial intensity of >50 mW/cm2 which is comparable with state-of-the-art Si-based emitters with much larger emission areas. Due to sub-wavelength confinement, the emission exhibits a high degree of spatial coherence, which is demonstrated by incorporating the light-emitting diode into a compact lensless in-line holographic microscope. This centimeter-scale, all-silicon microscope utilizes a single emitter to simultaneously illuminate ~9.5 million pixels of a complementary metal-oxide-semiconductor imager.


Microscope
The LEDs were characterized using the microscope shown in Supplementary Fig. 1.
The emission was collected using a 0.95NA objective and was routed either into an InGaAs camera for wide-field imaging or into a single-mode fiber (SMF) by flipping a mirror (M). Through the SMF, the emission was delivered into a photodiode or a spectrometer. A commercial 1100 nm LED was used as the illumination to take reflection micrographs.

Microscope transmission and camera calibration
The optical power transmission of the microscope was calibrated by backpropagating amplified spontaneous emission (ASE) of a semiconductor optical amplifier (SOA) from the collection SMF. An iris was used to match the beam size of the collimated ASE with the back aperture of the objective. The ASE spectrum is centered around 1130 nm, which is close to our emitter.
We measured the power of the ASE after the flipping mirror and after the objective. The optical power transmission is approximately 50.1% and 56.2% when the ASE is x-polarized and y-polarized, respectively. The polarization dependence is due to the two dichroic mirrors (not shown in Supplementary Fig. 1) between the objective and the flipping mirror. We used 53% as the average transmission to correct the emission power measured directly by the camera and the photodiode. Note that this correction does not compensate for the power loss from the fiber coupling and from the optics except the dichroic mirrors and the objective. The powers presented in the main text are therefore conservative estimations.
According to the factory test, the full well capacity and the ADC of our InGaAs camera are 1282000 and 16-bit, respectively, which corresponds to sensitivity of approximately 19.56 photo-electrons per digital count. The quantum efficiency is approximately 75% around 1100 nm. We thus can convert digital counts to photon numbers by 26.1 photons per digital count.

Gaussian fit and deconvolution
The images of the emission patterns were fit into two 2-D Gaussian functions with a constant.
where the first Gaussian denotes the n+/n emission spot and the other two terms denote the background. The fit area is 20 × 20 µm 2 . In the main text, we used the integral of the first Gaussian together with the optical power transmission and the camera sensitivity to estimate the n+/n emission power in Fig. 2 (e).
In Supplementary Fig. 3 The deconvolved FWHMs are used to estimate the emission area in Fig. 2 (h). At the interface of the poly-Si and the c-Si, a 2-dimensional heat source with 300 nm diameter and 13.5 mW power is set. We estimate an upper bound for heat generation by assuming that the voltage across the device -excluding the n-poly-Si contact -is all on the Si filament and the heat exchange from the carriers to the lattice happens all in the active region. Specifically, at 6 mA, the measured total voltage is 5.9 V, the calculated voltage drop on the n-poly-Si contact is 3.6 V based on its geometry and sheet resistance, and hence the estimated voltage across the nanoscale emitter is 2.3 V. We also assume the active region is the same size as the n+/n emission spot (0.09 ± 0.04 µm 2 at 6 mA).
The simulated temperature distribution in the proximity of the heat source is presented in Supplementary Fig. 5 (b). The temperature spatially decreases to room temperature within 1 µm from the heat source and the maximum local temperature rise is approximately 170 • C. Note that this is the upper limit of temperature rise in our device, which justifies that the substrate is an efficient heat sink.
As a comparison, we simulated an alternative device structure where SiO 2 is used to confine the current flow instead of a filament through the top oxide. In Supplemen- Here the maximum local temperature rise is approximately 400 • C, which is more than twice of the temperature rise in Supplementary Fig. 5 (b). We further increase the thickness of the STI to 0.5 µm and 1 µm for stronger carrier confinement, and the maximum local temperatures are approximately 550 • C and 780 • C, respectively. Strong Auger recombination and irreversible device degradation are likely to happen at these high temperatures.
The results above clearly show that efficient heat dissipation from the active region to a heat sink is required to ensure moderate local temperature rise. In our device, this is achieved by confining carriers using a local electrical field while leaving the heat conduction path to the substrate unaffected. The main limitation of the current design is that even though the lattice temperature rise is small the hot electrons injected from the n-poly-Si can still damage the device. This is likely the cause of the irreversible degradation at higher injection. We can further optimize the n-contact and fabricate shallow junctions near the active region to improve the reliability.  [17], 30 W/(m·K) [18], and 1.3 W/(m·K) [19], respectively.

Temperature-dependent device performance
We performed measurements on the SMF-coupled power with the chip die mounted on a temperature-controlled heat sink. (Supplementary Fig. 6.) We observe that the SMF-coupled power increases monotonically with temperature from 10 • C to 70 • C.
A similar trend of emission enhancement has been reported by Ng et al. [20] where the emission increases with temperature from 80 K to approximately room temperature. This trend suggests that Auger recombination, which is usually the main cause Even though the device favors an elevated temperature near room temperature, we emphasize that heat dissipation is essential because the emission enhancement is based on the mitigation of Auger recombination which increases significantly with temperature. Moreover, we notice that at 85 • C the optical power starts to decrease over time with 6 mA current, while the device is reliable with this current at 25 • C.
This suggests that breakdown site propagation, as a thermal runaway process, may happen with lower current at higher temperature.
Supplementary Fig. 6: Single-mode fiber (SMF)-coupled powers at multiple temperatures. The data points are normalized by the power with 6 mA and at 25 • C. The inset figure is the SMF-coupled power at 6 mA versus the heat sink temperature.

Variance and reliability
In Supplementary Fig. 7 The good reproducibility of the silicon filament formation has also been reported in the literature. For example, these Si filaments (usually referred as anti-fuses in analog circuit communities) can be arrayed precisely in standard CMOS platforms as one-time programmable read-only memory (OTP-ROM). [22,23] The variance of our device is probably due to the spatial randomness of the breakdown site within the contact area of the n-well and the gate oxide (≈ 0.3 µm 2 ). For example, if the breakdown happens near the interface of the n-well and the STI, a current leakage path may form. This is likely the situation of LED1 with the lowest SMF-coupled power since it also has the lowest forward bias voltage. We expect the reproducibility will be further improved with optimized design of the shape of the poly-Si contact.
In Supplementary Fig. 8, we plot the SMF-coupled power of LED2, which has intermediate performance in Supplementary Fig. 7 (a), after being turned on and off for ≈ 10 5 times as a reliability test. After the test, the SMF-coupled power decreased by approximately 25%. No significant optical power decrease was observed in the following measurements. As we mentioned in the main text, this is probably due to lateral propagation of the breakdown sites, which also leads to bias voltage decrease.
In the main text, we mainly present the device performance of LED2 after the reliability test. The TCSPC results are presented in Supplementary Fig. 9 (a, b). The optical pulses have distinct rise and fall edges. In Supplementary Fig. 9  Although the rise time to the steady state is relatively long, the time from 10% to 50% is only 4.7 ns, which is comparable to the fall time. The rising edge of the optical power corresponds to a 3-dB switching bandwidth on the order of 100 MHz. In Supplementary Fig. 9 (c), we present the average optical power when the LED is modulated by square waves with 50% duty cycle and 50% DC offset. The These bandwidths are one order of magnitude higher than those reported in [26].
The fast modulation is a result of the small active region and we expect higher bandwidths from future designs since our current device is not optimized for high speed modulation.
Supplementary Fig. 9: Time-resolved optical power and modulation speed.

Reference emitter
In Supplementary Fig. 10, the characterization results of the reference emitter are presented. Supplementary Fig. 10 (a) is a micrograph of the reference emitter biased at 0.9 mA taken by our microscope. Supplementary Fig. 10 (b) shows the emission pattern. The emission can be fit into a 2D Gaussian function of which the FWHMs are approximately 1 µm.
Supplementary Fig. 10 (c) shows the emission spectra at various currents. As discussed in the main text, compared with the LED, the reference emitter has a much broader emission spectrum, which indicates that the emission is mainly from impact ionization and hot carrier transition.